Paint Adhesion


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Q. Last year at this time we had a large amount of scrap due to paint adhesion failures. Our procedure for metal pretreatment was dip in alkaline cleaner, rinse in cold water and dip in a non-chromate final rinse. To solve this problem, I investigated other means of doing the job without going to an iron phosphate. I brought in a chemical supplier who suggested revamping the pretreatment line. The present procedure is alkaline cleaner, cold water rinse, iron phosphate, cold water rinse, non-chromate final rinse. Unfortunately, we still have adhesion problems.

My continued investigation led to the use of a wash primer. I made several tests with the two-package wash primer, spraying it over phosphated parts, parts that were just degreased, and parts which were pitted with rust. Midway through these tests, I decided to iron phosphate pretreat all parts, wash prime and then paint. In an attempt to find out if all these steps were necessary, I started experimenting, eliminating the iron phosphate and just using the wash primer. I have come to the conclusion that wash primer alone has done as good a job as iron phosphate.

Now my problem is to convince our engineering department that one of our procedures is not necessary. Needless to say, my preference is to do away with the iron phosphate. Any input you can give me as to the pros and cons of both methods would be appreciated. L.R.


A. Congratulations! You have confirmed one of the basic tenets of the painting industry, “Never apply two-part wash primers over pretreated metal.” The wash primer is a pretreatment and must be applied directly to clean metal so it can react with it and form an adhesive bond. Finally, don’t be too hasty in eliminating the iron phosphate pretreatment. The problem is that use of two-part wash primers which contain zinc chromate pigments may not be available everywhere and perhaps not at all. In that case, using the iron phosphate pretreatment is a better approach.